For a contact lens to be effective, it should be reasonably well centered on the wearer's eye. In practice, each contact lens slides around and rotates on the surface of the eye as the observer blinks or redirects his or her direction of gaze. Therefore, the lens should be designed to quickly re-center itself after each blink and to remain reasonably well centered as the subject changes their direction of gaze. As part of the lens design process, the contact lens manufacturer conducts clinical studies of lens position dynamics. This ensures that the lens meets positional and movement requirements before it is produced and sold to end users.
Moreover, lenses are manufactured with a variety of base curvatures in order to accommodate variations in the size (corneal curvature) of individual eyes. Contact lenses are therefore “fitted” to each patient's eye. The fitting process can involve placing lenses with different base curves in the patient's eye and studying the position dynamics of the lens relative to the eye.
Prior approaches to tracking position and movement of contact lenses on a subject wearer's eye involve the use of video-capture equipment in combination with human analysts. In order to support the study of lens dynamics in research and clinical settings, one approach is to employ a color camera to acquire a video sequence of a subject wearer's eye while wearing a contact lens. A semi-automated procedure is then used to analyze the video sequence. During the semi-automated analysis, the analyzing clinician views individual frames of the video sequence on a computer display and manipulates a mouse or other user interface device to define multiple points along (1) the edge of the pupil, (2) the edge of the limbus (i.e., outer edge of the iris pattern), and (3) the edge of the contact lens. This procedure is repeated for a sampling of video frames following each eye blink. The system then fits a circle to each set of manually-entered points, and the fit circle centers for pupil, limbus, and contact lens are saved for each analyzed frame. Finally, this position data is later loaded into a spreadsheet or other formatted data file for further visualization and analysis. In particular, the offset between the center of the lens and the center of the pupil is often computed and analyzed with respect to time, as is the offset between the center of the lens and the center of the limbus.
This prior art approach typically employs broad-spectrum, visible illumination and an RGB color camera to acquire the color video sequence that is analyzed. Although the lens is transparent over the entire visible wavelength spectrum, the lens edge is generally only barely visible in the acquired images. During analysis, the most effective technique for identifying the lens edge is through human observer (typically the clinician), who can use the edge information to carefully define points on the edge of the lens. The clinician can also use these techniques to define the location of the pupil or limbus for a relative eye position. Once the edge location is manually defined, the system can then use conventional curve-fitting algorithms to fit circles to define these points. The circles allow for the generation of a center that is thereby used to track the position of the lens on the eye.
Disadvantageously, this semi-automated approach is very labor intensive and prone to human operator error. While it has the potential to significantly reduce labor and increase accuracy, the use of machine vision to locate the contact lens edge is problematic. This is because a contact lens is usually designed to be as invisible and undetectable as possible on the wearer's eye. Thus, the use of visible, broad-spectrum light does not produce an image that is reliably resolved by a vision system. An alternate approach, using machine vision, is to illuminate and image a subject wearer's eye using a contact lens that has been provided with one or more visible fiducials, along with fiducials applied to the wearer's eyelid(s) as a reference. This approach involves extra discomfort to the subject wearer as fiducials must be applied to a sensitive region. Moreover, the visible fiducials on the contact lens may cause the wearer to react differently than normal as the fiducial(s) potentially move into and out of the wearer's field of view during blink-induced lens movement.
More generally, the use of visible light to illuminate the eye during video capture is uncomfortable to the wearer. Often, relatively bright lights are needed to adequately resolve the contact lens edge. This also potentially effects the reactions of the wearer (for example, causing excessive blinking), reducing the accuracy and realism of the test results.
It is therefore desirable to provide a system and method that effectively employs a vision system to accurately and reliably track lens position and movement on a subject wearer's eye. This system and method should eliminate the need for fiducials in the determination of lens position and movement (typically on a spherical lens), and reduce or eliminate the need for bright visible-spectrum illumination and the associated discomfort it causes to the subject wearer.